In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, mobile terminals, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a base station, a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals within range of the base stations.
Cellular communications system operators have begun offering mobile broadband based on WCDMA (Wideband Code Division Multiple Access) and/or HSPA (High Speed Packet Access). Moreover, fuelled by introduction of new devices designed for data applications, end user performance requirements are steadily increasing. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by HSPA networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently may be desired.
Techniques to improve downlink performance may include 4-branch MIMO (Multiple Input Multiple Output), multiflow communication, multi carrier deployment, etc. Since spectral efficiencies per link may be approaching theoretical limits, next steps may include improving spectral efficiencies per unit area. Further efficiencies for HSDPA may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell. Currently, heterogeneous networks are being developed for 3GPP as discussed, for example, in: RP-121436, Study on UMTS Heterogeneous Networks, TSG RAN Meeting #57, Chicago, USA, 4-7 Sep. 2012; R1-124512, Initial considerations on Heterogeneous Networks for UMTS, Ericsson, ST-Ericsson, 3GOO TSG RAN WG1 Meeting #70bis, San Diego, Calif., USA, 8-12 Oct. 2012; and R1-124513, Heterogeneous Network Deployment Scenarios, Ericsson, ST-Ericsson, 3GPP TSG-RAN WG1 #70bis, San Diego, Calif., USA, 8-12 Oct. 2012.
A homogeneous network is a network of base stations (also referred to as NodeB's) in a planned layout providing communications services for a collection of user terminals (also referred to as user equipment nodes, UEs, and/or wireless terminals) in which all base stations may have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all base stations in a homogeneous network may offer unrestricted access to user terminals in the network, and each base station may serve roughly a same number of user terminals. Current cellular wireless communications systems in this category may include, for example, GSM (Global System for Mobile communication), WCDMA, HSDPA (High Speed Downlink Packet Access), LTE (Long Term Evolution), Wimax (Worldwide Interoperability for Microwave Access), etc.
In a heterogeneous network, low power node base stations (also referred to as low power nodes, LPNs, micro nodes, pico nodes, femto nodes, relay nodes, remote radio unit nodes, RRU nodes, small cells, RRUs, etc.) may be deployed as shown in FIG. 1 together with planned and/or regularly placed macro base stations, including macro base station MBS. Macro base station MBS may thus provide service over a relatively large macro cell area Mca, and each LPN may provide service for a respective relatively small LPN cell area Lca within the relatively large macro cell area Mca. Power transmitted by an LPN (e.g., 2 Watts) may be relatively small compared to power transmitted by a macro base station (e.g., 40 Watts for a typical macro base station). An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in macro cell area Mca of macro base station MBS and/or to off-load traffic from macro base station MBS (e.g., to increase capacity in a high traffic location, also referred to as a hot-spot). Due to the lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition(s).
LPNs deployed in a heterogeneous network may have a property that each LPN has its own cell identity (e.g., a unique scrambling code). More particularly, LPNs within macro cell area Mca and macro base station MBS servicing the macro cell area Mca may operate as different cells, but the LPNs and MBS within macro cell area Mca may share the same frequency, and this arrangement may be referred to as co-channel deployment with each LPN and MBS having a unique cell identity (e.g., a unique scrambling code).
FIG. 2 illustrates a heterogeneous network with co-channel deployment, where the low power node cell areas Lca-1 and Lca-2 (also referred to as low power cells) may be serviced by respective low power nodes LPN-1 and LPN-2 as well as macro base station MBS providing service for the larger macro cell area MCA (also referred to as a macro cell). Note that each base station (i.e., each of MBS, LPN-1, and LPN-2) may provide service using a respective individual pilot signal (e.g., CPICH-MBS, CPICH-1, and CPICH-2), downlink control channels, uplink control channels, and data traffic channels. Stated in other words, each base station may act independently so that cell areas (Mca, Lca-1, and Lca-2) or cell may be characterized by respective pilot signals, downlink control channels, uplink control channels, and data traffic channels. Accordingly, each base station may provide service using a different common pilot channel (CPICH).
FIG. 3 is a graph illustrating simulated gains achieved in average sector throughput with different values of LPN power. Note that a maximum gain may be achieved when the cells are fully loaded, and the gains may be primarily achieved due to load balancing.
Even though gains in average sector throughput may be provided by deploying low power nodes LPNs, throughput of individual UEs communicating with low power nodes LPNs may be impacted due to interference from macro base station MBS transmissions. Such MBS interference may be most significant when a UE is connected with a low power node LPN and the dominant interferer is the macro base station MBS. The region of a LPN cell area Lca where this type of interference from a macro base station is most prevalent may be referred to as an LPN cell expansion region or Lcer. A cell expansion range Lcer area of low power node LPN is illustrated in FIG. 4 as an outer region of LPN cell area Lca. In this cell range expansion area Lcer, a UE may experience relatively strong interference from macro base station MBS transmissions. In some cases, a UE in the cell expansion range Leer area may receive MBS transmissions at greater power than transmissions from LPN. By forcing connection to the LPN when a UE is in a cell range expansion area, however, overall network performance may be improved by unloading traffic from the macro base station MBS.
FIG. 5 is a graph illustrating simulated link throughput when a UE is connected to an LPN with relatively strong interference from a macro base station (e.g., when the UE is located in the LPN cell expansion region Lcer of FIG. 4). As shown in FIG. 5, UE performance may be significantly impacted when the dominant interferer power received at the UE is 10 to 20 times greater than that of low power node transmissions received at the UE. Stated in other words, performance of UE reception may be significantly impacted when the UE is located in an LPN cell expansion area where MBS transmissions received at the UE have a power that is at least 10 times greater than a power of LPN transmissions received at the UE.